There are numerous applications in which it is necessary to rapidly turn on and turn off high-current capacity power switches to control the power delivered to a load. For example, in an inverter operated in a pulse-width modulated mode of operation, it may be necessary to cycle power switches carrying a high magnitude of current at a high switching frequency. Typically, although not necessarily, these power switches comprise insulated gate bipolar transistors (IGBT's). A gate drive for an IGBT provides the high-current pulses required to turn the IGBT on and off and also provides electrical isolation between the IGBT and control circuits. The electrical isolation can be achieved through the use of an optocoupler or a transformer. A drive circuit using a transformer has the advantage that power can be conveyed across the transformer to operate the IGBT. In contrast, optically based designs typically require a local power source for each IGBT to be controlled.
Nollet, U.S. Pat. No. 4,767,952 discloses a high-speed control circuit for power field effect transistors. Control signals are transmitted via a transformer to drive circuitry. The drive circuitry includes rectifiers for half-wave rectifying the control pulses and a pair of parallel-connected capacitors coupled to the rectifiers. Control electrodes of a pair of power transistors are coupled to the secondary winding of the transformer. The sources of the power transistors are in turn coupled to a control electrode of an output transistor. When a pulse is developed across the secondary winding of the transformer, one of the power transistors is immediately turned on and current is either supplied to or removed from the control electrode of the output transistor by one of the capacitors. The capacitor is recharged during a subsequent portion of the pulse so that the local power supply is restored.
Other patents disclosing drive circuitry for power switches include Seager, U.S. Pat. No. 3,697,783; Inami, et al., U.S. Pat. No. 3,971,961; Baker, U.S. Pat. No. 4,210,826; Carlsen II, U.S. Pat. No. 4,234,805; Akamatsu, U.S. Pat. No. 4,239,988; Coates, U.S. Pat. No. 4,259,939; Shelly, U.S. Pat. No. 4,423,341; Stefani, U.S. Pat. No. 4,636,713; Wheeler, U.S. Pat. No. 4,654,544; Okado, U.S. Pat. No. 4,721,869; Shekhawat, et al., U.S. Pat. No. 4,725,741; Jessee, et al., U.S. Pat. 4,728,817; Shekhawat, et al., U.S. Pat. No. 4,947,055 and Japanese Patent Publication No. 60-244120.